In the long term evolution (LTE) of 3GPP, a new flexible air interface is currently being standardized. The LTE system will provide spectrum flexibility in the sense that carrier bandwidths varying between 1.25 MHz and 20 MHz may be handled, and both Frequency Division Duplex, FDD, and Time Division Duplex, TDD, is supported in order to be able to use both paired and unpaired spectrum. LTE is expected to be smooth evolution path for 3G standards such as WCDMA, TD-CDMA and TD-SCDMA. LTE is also expected to offer significant performance improvements as compared to current 3G standards by using, for example, various advanced antenna techniques.
In the downlink of LTE, the physical layer is based on Orthogonal Frequency Division Multiplex, OFDM. Here the information to be transmitted is coded, for example, by using turbo codes, interleaved, modulated to signal constellations of various orders, for example, QPSK, 16 QAM, 64 QAM or the like, and fed to an inverse Fast Fourier Transform (IFFT). This IFFT is a transform from a frequency domain representation of the symbols to be transmitted to a time domain representation.
In the uplink of LTE, the physical layer is based on Single Carrier-Frequency Division Multiple Access, SC-FDMA, which is also referred to as pre-coded OFDM. This means that the physical channels are built of SC-FDMA symbols. Here, the modulated symbols are transformed to the frequency domain by a Discrete Fourier Transform, DFT, of the same size as the number of modulated symbols of each SC-FDMA symbol. This is then fed to a larger IFFT with a size which depends on the bandwidth of the radio communication link.
This way, several users may be scheduled to transmit simultaneously, typically on different parts of the frequency band in a TDMA/FDMA fashion.
In the receivers of downlink and uplink the received signal is fed to a Fast Fourier Transform, FFT. This FFT enables a frequency domain extraction of each user such that the symbols for each user may be demodulated and decoded independently.
Whenever a DFT (implemented as an FFT) is used by a base station receiver on a time domain signal, said time domain signal is assumed to be periodic with period time equal to the length of the DFT. In order to ensure this for signals subject to dispersive channels, a cyclic prefix may be used. The cyclic prefix must have a length that is larger, or equal to, the delay spread of the uplink communication channel.
All these FFT:s are preferable implemented with fixed point calculations, such that the calculations can be done in cheap hardware with low power consumptions.
However, when an FFT/IFFT is calculated with fixed point calculations, a quantization noise occurs in the result. This quantization noise is larger in some regions compared to others and may deteriorate the performance of system.